Sea surface temperature ATSR

The surface temperature of the Earth's oceans is a key climate parameter and also a tracer for warm and cold currents near the surface. Again, rather precise measurements are required, with precisions of <0.1 K and absolute accuracies of better than 0.5 K. Achieving these fairly demanding values from space is made particularly difficult by the presence of the intervening atmosphere, which has absorption features even in the most transparent spectral windows, plus very variable attenuation due to thin clouds and aerosols.

A recent instrument designed to surmount these difficulties by using two views of the surface at an angle of 60° to each other, is the Along-Track Scanning Radiometer (ATSR). The radiance measured from the same footprint from different points in the orbit is composed of essentially the same component from the surface (assuming the surface emissivity is independent of direction, which is generally true to a good approximation), but with the atmospheric absorption and emission weighted by the cosine of the zenith angle as a result of the longer path length. ATSR has channels in the windows at 3.7, 10.8 and 12.0 ¡m (Fig. 10.5). Within the instrument, the direction of view is actually swept by a rotating mirror through a cone (Fig. 10.6), skewed so that one side is in the local vertical to give the nadir view. The functions of modulation and calibration are combined by employing a sufficiently rapid scan that encompasses the scene and the two standard sources, producing a video-type output. This approach eliminates the effect of background drift, as well as the need for a separate

Wavelength (|j,m)

Wavelength (|j,m)

flg. 10.5. The filter bandpasses that define the ATSR spectral channels are shown at the top; a response of 1.0 means the filter is perfectly transmitting at that wavelength while 0 means it is opaque. Perfectly square filters are difficult to manufacture so real instrument profiles have complex shapes as shown here. The bottom panel shows the transmission of the atmosphere, from the sea surface to the satellite, as a function of wavelength. Comparing the upper and lower panels shows how the filters select radiation from the surface in spectral 'windows', i.e. regions of high transparency, so the obscuring effect of the atmosphere and the contribution emission from the atmosphere makes to the signal are minimized.

chopper and calibration mirror. It also has the multiple advantages of scanning from side to side of the subspacecraft track, of allowing views of two calibration blackbodies positioned between the downward- and forward-viewing apertures every time the Earth-viewing direction is switched, and of accomplishing the switching quickly and smoothly. Otherwise, ATSR is conceptually a fairly conventional modern infra-red radiometer, using interference filters to define the spectral bands and closed-cycle refrigerators to cool the HgCdTe detectors.

The key to success in measuring the subtle effects of ocean temperature on the climate system, and in particular to have hope of measuring secular changes, is careful calibration of the instrument before flight. Figure 10.7 shows the arrangement whereby ATSR was geometrically and radiometrically calibrated under vacuum. The field of view measurement used a hot point source in the form of a tungsten ribbon filament, capable of temperatures up to 3200 K, illuminating a pinhole of diameter 70 ¡m in the focal plane of a fixed offaxis paraboloid, with a two-axis gimbal-mounted mirror in the configuration shown in Fig. 10.7 to provide a scanning, collimated input beam. The size of the mirrors was such as to completely fill the aperture of the space instrument during testing, which is essential since the optical train is not symmetric about its axis.

For radiometric calibration, two conical blackbodies were used, arranged so that

flg. 10.6. Schematic of the Along-Track Scanning Radiometer. The filters described in Fig. 10.5 are in the housing with the detectors, which is cooled to 77 K by a miniature Stirling-cycle refrigerator. The scan mirror is spun by a motor, and is tilted, as shown, with respect to the rotation axis so that the view seen by the detectors describes a circle. Within that circle it scans across (i) a nadir (vertically downwards) view of the Earth, (ii) an along-track view at 53° to the vertical along the orbit track, so that the same area is seen twice about 2.5 minutes apart as the spacecraft moves in its orbit; (iii) a cool blackbody (inside the page in this diagram) and (iv) a warm blackbody (above the page). The blackbodies are kept at temperatures representative of the upper and the lower values of the sea-surface temperature (265 and 305 K).

both were viewed simultaneously through the two Earth-viewing ports, and mounted on a rotating plate that allowed the two calibration targets, at different temperatures representing the range expected from the Earth, to be interchanged during the course of a test run. Calculations established the emissivity of the reference blackbodies to be 0.9985 at 3.7 pm and 0.9995 in the longer-wavelength channels. Other practical details of importance are the complex system of blackened, temperature-controlled baffles that represent the operating environment of the instrument when in space by simulating the radiative background due to the spacecraft, the Earth, and cold space.

Vacuum chamber

flg. 10.7. For pre-flight laboratory calibration, the Along-Track Scanning Radiometer (ATSR) was installed in a vacuum chamber at an angle so it could view two standard radiation sources (blackbodies) BB1 and BB2 at different temperatures that span the range of expected target temperatures on the Earth. The internal scan mirror of the instrument views the onboard calibration blackbodies and the standard sources in the chamber in rapid succession. The sources in the chamber are of superior construction to those inside the instrument, but are themselves too bulky to be carried into space. Hence they are used to cross-calibrate the simpler inflight blackbodies and determine their emissivity. BB1 and BB2 are mounted on a rotating plate that allows them to be interchanged, so that any radiometric differences between the downward and along-track views can be assessed. The inside of the chamber has liquid-nitrogen-cooled walls and other thermally controlled surfaces to simulate conditions on the satellite in space.

flg. 10.7. For pre-flight laboratory calibration, the Along-Track Scanning Radiometer (ATSR) was installed in a vacuum chamber at an angle so it could view two standard radiation sources (blackbodies) BB1 and BB2 at different temperatures that span the range of expected target temperatures on the Earth. The internal scan mirror of the instrument views the onboard calibration blackbodies and the standard sources in the chamber in rapid succession. The sources in the chamber are of superior construction to those inside the instrument, but are themselves too bulky to be carried into space. Hence they are used to cross-calibrate the simpler inflight blackbodies and determine their emissivity. BB1 and BB2 are mounted on a rotating plate that allows them to be interchanged, so that any radiometric differences between the downward and along-track views can be assessed. The inside of the chamber has liquid-nitrogen-cooled walls and other thermally controlled surfaces to simulate conditions on the satellite in space.

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